The present invention relates to a mixture comprising a nematic liquid crystal and a non-mesomorphic compound. More specifically, it relates to a mixture comprising a nematic liquid crystal usable in devices involving the electrically controlled birefringence effect.
In certain liquid crystal devices, the electrooptical effect used is electrically controlled by refringence (ECB). This effect corresponds to a deformation under an electric field of a nematic phase with negative dielectric anistropy .DELTA..epsilon., the latter representing the difference between the dielectric constant .epsilon..sub.a parallel to the major molecular axis of the crystal and the dielectric constant .epsilon..sub.b perpendicular to said major axis.
In liquid crystal devices, it is necessary to use nematic materials permitting a high multiplexing level, i.e. a large number of electrically addressable screen lines, in order to display a high information level. This multiplexing level k can be expressed as a function of the voltage applied to the terminals of the device (V) and the deformation threshold voltage of the crystal (V.sub.S) by the formula: ##EQU1##
For a given multiplexing level, it is necessary to have the largest possible molecular tilt angle (.PHI..sub.M) in the centre of the liquid crystal cell, in order to obtain the optimum contrast between the two black and white states of the crystal under the action of the electric field. For small tilt angles .PHI..sub.M is directly linked with the deformation threshold voltage of the crystal V.sub.S and the ratio of the bending K.sub.33 and fanning K.sub.11 elastic constants of the liquid crystal. Thus, .PHI..sub.M is given by the formula: ##EQU2##
Under these conditions, the electrically induced birefringence is then of the form: ##EQU3## in which n.sub.e and n.sub.o are the extraordinary and ordinary indices of the crystal, e the liquid crystal thickness and Z the position of the crystal in the cell.
The relative intensity transmitted by the liquid crystal device between two cross-polarizers is given by the relation: ##EQU4## in which .DELTA.n is equal to n.sub.e -n.sub.o and .lambda. is the wavelength of the light beam illuminating the device. Thus, the most important parameters of the material are (K.sub.33 /K.sub.11), .DELTA.n, .DELTA..epsilon., as well as the crystal mesomorphism range .DELTA.T.
An important function is played by the ratio K.sub.33 /K.sub.11, because it determines the steepness of the slope of the electrooptical transfer curve and therefore the multiplexibility of the material used. This ratio must be as large as possible.
As a function of the nematic compounds used, this ratio can vary from 0.5 to 3, the most widely used value being around 1. In order to have a higher value of the ratio K.sub.33 /K.sub.11, consideration can be given to increasing the value of K.sub.33 or decreasing the value of K.sub.11.
A high value for K.sub.33 is of no interest in display devices, because it involves the use of high control voltages. Thus, the deformation threshold voltage of the crystal V.sub.s is proportional to the value of the bending elastic constant K.sub.33 according to formula: ##EQU5##
It is therefore preferable to decrease the value of K.sub.11, which corresponds to decreasing the intermolecular interactions between the molecules of the liquid crystal. Generally the nematic liquid crystal molecules are formed from a rigid central part having phenyl groups and one or more flexible parts located at the ends of the rigid part.
Several solutions have been considered for reducing the interactions between the molecules of this type. One of these solutions is to replace a phenyl cycle of the rigid part by a cyclohexane or by a bicyclo-octane nucleus. Thus, the latter nucleus as a result of its substantially globular shape and its completely saturated bonds does not aid intermolecular interactions and this makes it possible to reduce the value of K.sub.11 and increase the ratio K.sub.33 /K.sub.11. A second solution is to substitute one or more hydrogen atoms of the phenyl nucleus by an electronegative element such as fluorine.
However, such modifications suffer from the disadvantage of leading to molecules which do not all have the requisite electrooptical properties for display devices. It is therefore necessary to use them mixed with other liquid crystals, so that there is a significant increase in the value of the K.sub.33 /K.sub.11 ratio.